Marine scientists typically look at coral cover to decipher whether a reef is thriving. But by the time corals begin to diminish, it may already be too late.

A new study, however, offers a different approach. In order to detect earlier symptoms of future coral catastrophe, WCS researchers and others suggest looking at the ecosystems’ aquatic residents, specifically how many kilograms (kg) of fish the reef has per hectare (ha). Conducted by WCS scientists and partners in the Western Indian Ocean, the study illustrates how loss of fish causes reef quality to degrade in eight stages. Fishery managers will be able to use the research to decide when to restrict fishing practices around the reef.

“The study identifies eight changes before all of the ecological lights go off and the reef and fishery is gone,” said Dr. Tim McClanahan, the lead author on the study and the head of WCS’s coral reef research and conservation program. “Below 300 kilograms per hectare we see a series of dramatic changes on reefs. This is where you get on a real slippery slope. Strangely, the metric used by most managers to gauge the health of reef systems—coral cover—is the last threshold before ecosystem failure. Overfished reefs can appear healthy and then shift to algae dominated seascapes.”

In well-protected areas, the scientists typically found 1000to 1500 kg/ha of reef fish of various species. But when the fish biomass dipped below 1000 kg/h, the researchers began seeing spurts in seaweed growth and urchin activity. Under 300 kg/ha, the coral ecosystem finds itself in very troubled—and largely fish-free—waters.

“The good news is that a reef can likely provide sustainable fisheries even after the first three warning switches are turned off, but it becomes increasingly difficult to maintain a healthy fishery and restore reefs when the final five switches have been turned off,” said Dr. McClanahan. “This study provides managers and policymakers with a tangible target of where to maintain their fishery.”

The study found that reef fisheries without regulations tended to do poorly. Some hit all the switches and completely collapsed. No-take marine reserves, where fishing is banned, were the healthiest, tending to maintain their predators and key ecosystem processes.

“People depend on reefs for their livelihoods, so we can’t prohibit fishing everywhere,”noted Dr. Joshua Cinner of Australia’s James Cook University. “A key finding from our study was that even easily enforceable regulations that restrict gear or the types of species that can be caught helped maintain biomass. These regulations are often more agreeable to fishermen than no-take closures and consequently, receive higher levels of support and compliance.”